Issue #16400: Add command line option for isolated mode.

-I

    Run Python in isolated mode. This also implies -E and -s. In isolated mode
    sys.path contains neither the script’s directory nor the user’s
    site-packages directory. All PYTHON* environment variables are ignored,
    too. Further restrictions may be imposed to prevent the user from
    injecting malicious code.

Modules/main.c

@@ -43,7 +43,7 @@ static wchar_t **orig_argv;
 static int  orig_argc;
 
 /* command line options */
-#define BASE_OPTS L"bBc:dEhiJm:OqRsStuvVW:xX:?"
+#define BASE_OPTS L"bBc:dEhiIJm:OqRsStuvVW:xX:?"
 
 #define PROGRAM_OPTS BASE_OPTS
 
@@ -65,6 +65,7 @@ Options and arguments (and corresponding environment variables):\n\
 static char *usage_2 = "\
 -i     : inspect interactively after running script; forces a prompt even\n\
          if stdin does not appear to be a terminal; also PYTHONINSPECT=x\n\
+-I     : isolate Python from the user's environment (implies -E and -s)\n\
 -m mod : run library module as a script (terminates option list)\n\
 -O     : optimize generated bytecode slightly; also PYTHONOPTIMIZE=x\n\
 -OO    : remove doc-strings in addition to the -O optimizations\n\
@@ -426,6 +427,12 @@ Py_Main(int argc, wchar_t **argv)
             Py_InteractiveFlag++;
             break;
 
+        case 'I':
+            Py_IsolatedFlag++;
+            Py_NoUserSiteDirectory++;
+            Py_IgnoreEnvironmentFlag++;
+            break;
+
         /* case 'J': reserved for Jython */
 
         case 'O':